As is known in the art, a microstrip antenna typically includes a substrate having a ground plane disposed on a first surface thereof and a radiating element provided as a strip conductor having a rectangular or circular shape disposed on a second surface thereof. Radio frequency (RF) energy is typically coupled to the radiator via an RF feed circuit. When a single RF feed circuit is coupled to a radiating element having a rectangular shape, the antenna radiates electromagnetic energy having a linear polarization.
As is also known, there has been a trend in microstrip antennas to provide the substrate as a ferrite substrate and provide a DC magnetic field perpendicular to the plane in which the radiating element is disposed. This is done to provide the antenna having phase shifting, impedance matching and frequency tuning capabilities. For example, by applying a DC magnetic field to a microstrip antenna array having a plurality of radiating elements disposed over a ferrite substrate, a so-called main antenna beam of the antenna may be scanned and the radiation frequency of a microstrip antenna may be tuned by varying the strength of the biasing magnetic field.
One problem with such rectangular shaped radiating elements having a single feed however, is that they are linearly polarized. In many applications, it would be desirable to be able to receive RF energy of any polarization, and in particular it would be desirable to receive RF energy having two orthogonal polarizations or circular polarizations.
Microstrip antennas having circular polarization are able to receive RF energy having any polarization. Conventional circularly polarized microstrip antennas include a radiating element having a circular shape and a single feed circuit (i.e., single feed circular patch). Alternatively, a radiating element having a rectangular shape fed from two properly phased feed circuits coupled to adjacent sides of the radiating element (i.e., dual feed rectangular patch) may also provide circular polarization. In the single feed circular patch approach, the amount of elipticity in the shape of the radiating element determines the mixing of the two orthogonal linearly polarized radiations which in turn provides the circularly polarized waves.
In the dual feed rectangular patch approach, two orthogonal feeds are required in which the relative phases are shifted 90.degree. apart from each other to provide an antenna which may radiate electromagnetic energy having circular polarization.